1. Field of the Invention
The present invention generally relates to the conversion of chemical energy to electrical energy, and more particularly, to an alkali metal electrochemical cell having a transition metal oxide cathode activated with a nonaqueous electrolyte. The transition metal oxide cathode active material is preferably silver vanadium oxide having a single phase with a surface area of about 0.2 m2/gram to about 0.80 m2/gram. The single phase silver vanadium oxide is produced in a decomposition reaction at a final decomposition temperature of about 490xc2x0 C. to about 520xc2x0 C.
2. Prior Art
U.S. Pat. Nos. 4,310,609 and 4,391,729 to Liang et al. disclose the preparation of silver vanadium oxide (SVO) as a cathode material for use in a nonaqueous electrolyte battery. These patents describe the preparation of silver vanadium oxide by a thermal decomposition reaction involving a final heat treatment step of about 360xc2x0 C.
U.S. Pat. No. 4,830,940 to Keister et al. describes a solid cathode, liquid organic electrolyte, lithium cell for delivering high current pulses. The solid cathode includes as an active material AgxV2Oy wherein x is in the range from about 0.5 to about 2.0 and y is in the range from about 4.5 to 6.0. Keister et al. reference the publication xe2x80x9cEffect of Silver Content On the Performance of Primary Lithium/Silver Vanadium Oxide Batteriesxe2x80x9d, E. S. Takeuchi and P. Keister, Electrochemical Society, Oct. 13-18, 1985, Las Vegas, Nev., Abstract No. 125, which describes the preparation of silver vanadium oxide at about 360xc2x0 C. from the thermal decomposition of silver nitrate and vanadium pentoxide.
U.S. Pat. No. 5,221,453 to Crespi discloses the preparation of silver vanadium oxide by a chemical addition reaction (combination of AgVO3 and V2O5 or Ag2O and V2O5) in a temperature range of about 300xc2x0 C. to about 700xc2x0 C. The chemical addition reaction is described as being distinct from the thermal decomposition reaction described by Liang et al. and Keister et al. A decomposition reaction is characterized by the evolution of nitrogen oxide gas when the reactants are V2O5 and AgNO3. A chemical addition reaction does not include the evolution of reaction by product gases.
In the publication R. A. Leising and E. S. Takeuchi, Chemistry of Materials, 5, 738-742 (1993) the preparation of silver vanadium oxide by the thermal decomposition of AgNO3 and V2O5 at temperatures of 320xc2x0 C., 375xc2x0 C., 450xc2x0 C. and 540xc2x0 C. is described. This publication also reports discharge results of experimental lithium cells containing silver vanadium oxide cathode active materials prepared at those various temperatures and activated with 1M LiAsF6 PC/DME electrolyte. The 375xc2x0 C. prepared SVO material gave slightly higher delivered capacity than the 450xc2x0 C. material, and significantly higher capacity than the SVO material prepared at 540xc2x0 C. As an illustration, the graph in FIG. 1, curve 10, was constructed from the discharge capacity versus synthesis temperature of lithium cells having silver vanadium oxide prepared in a thermal decomposition reaction of AgNO3 and V2O5 according to this publication. The delivered capacity of the variously prepared cells was measured using a constant resistance discharge over a short period of time (less than 2 days).
The present invention relates to a nonaqueous electrolyte, alkali metal/transition metal oxide electrochemical cell and, in particular, a lithium/silver vanadium oxide electrochemical cell designed for high current pulse discharge applications while exhibiting reduced or no appreciable voltage delay and reduced Rdc build-up. An example of such an application is an implantable cardiac defibrillator, where the battery may run under a light load, device monitoring mode for extended periods of time interrupted by high rate, current pulse discharge during device activation. Voltage delay is a phenomenon typically exhibited in a lithium/silver vanadium oxide cell that has been depleted of about 40% to about 70% of its capacity and is subjected to current pulse discharge applications. The occurrence of voltage delay is detrimental because it may result in delayed device activation and shortened device life.
The desirable decrease in voltage delay is realized in lithium cells that, according to the present invention, contain a low surface area, single phase silver vanadium oxide prepared in a decomposition reaction at a temperature above the decomposition temperature of the mixture of starting materials followed by a second heating at a temperature in the range of about 490xc2x0 C. to about 520xc2x0 C. and are activated with a nonaqueous electrolyte.
A particularly preferred transition metal oxide cathode active material comprises a single phase silver vanadium oxide having the general formula AgxV2Oy wherein in the xcex5-phase x=1.0 and y=5.5. According to the present invention, this material is produced in a decomposition reaction by first heating the mixture of starting materials to a temperature above the decomposition temperature of the mixture followed by a second heating at a final heating temperature of about 490xc2x0 to about 520xc2x0 C. This particularly preferred SVO material is in contrast to xcex2-phase silver vanadium oxide having in the general formula x=0.35 and y=5.18 and xcex3-phase silver vanadium oxide having in the general formula x=0.74 and y=5.37. The surface area of this material is about 0.2 m2/gram to about 0.80 m2/gram.
A typically used electrolyte for activating this electrochemical couple comprises 1M LiAsF6 dissolved in a 50:50 mixture, by volume, of PC and DME. The increase in usable cell capacity, and subsequent increase in battery life in such a cell is unexpected based on the published capacity data for 450xc2x0 C. and 540xc2x0 C. SVO discharged under quick discharge conditions. Again, in the previously discussed Leising et al. publication, the 450xc2x0 C. SVO gave less capacity than SVO prepared at 375xc2x0 C., and it could be inferred that the further the preparation temperature is from 375xc2x0 C., the less capacity the cell will provide.